1. FIELD OF THE INVENTION
The invention pertains to an injection molding unit for an injection molding machine. The injection unit cooperates with a mold and is supported on a stationary machine pedestal. The injection unit includes a pair of bridges capable of moving on guide rods, one of the bridges carrying a plastification cylinder with a plastification screw therein. The injection unit also includes an injection cylinder for advancing the plastification screw in an injection stroke along an injection axis and hydraulic drive cylinders cooperating with drive pistons on the guide rods to drive the injection unit.
2. DESCRIPTION OF THE PRIOR ART
Known injection units of this type are disclosed, for example, in German Offenlegungsschrift Nos. 20 20 337; 30 13 426 (which corresponds with U.S. Pat. No. 4,293,229); 29 07 556--FIG. 2 (which corresponds with U.S. Pat. No. 4,251,204); and German Auslegeschrift No. 1 961 819 (which corresponds with U.S. Pat. No. 3,729,280). Such injection units are supported exclusively by their guide rods, that is, by fixed parts. In these cases the supporting elements, in particular support brackets, fit into the rear ends of the guide rods, such that in each case the axial distance between the front and rear support points on the guide rods is large enough to enable the drive cylinder and injection cylinder or cylinders to slide forward and/or back.
In the injection unit as described by German Offenlegungsschrift No. 20 20 337, a glide footing is additionally attached to the bridge which carries the plastification cylinder. However, when the injection unit is in the operating position, it is located at some distance from the top of the plastic injection molding machine's pedestal. This glide footing can thus support the injection unit only when it is retracted from its operating position. It is therefore no more than an aid for use in retracting the injection unit.
It has been shown that, when the known injection units under consideration are subjected to intensive use over long periods of time, precision guiding deteriorates, which is clearly the result of the periodic bending of the guide rods under the load of the injection unit during injection cycles. This then becomes a disadvantage, for example, when the nozzle of the injection unit forms part of the confines of the injection mold cavity.